题目

Given an array of integers, return indices of the two numbers such that they add up to a specific target.

You may assume that each input would have exactly one solution.

Example:

Given nums = [2, 7, 11, 15], target = 9,Because nums[0] + nums[1] = 2 + 7 = 9,return [0, 1].

解题思路

我想大多数人和我一样都会想到用这种方法。写两个for循环，依次查找看A[i] + A[j] 是否等于target.若等于返回i，j.

思路2：对于java 我们可以采用Map（以空间换时间）时间复杂度为O(n)。

通过map来查找a和target-a是不是都在数组中,如果在则返回他们的下标。

代码：

import java.util.*;public class Solution {//思路1    public int[] twoSum(int[] nums, int target) {        int[] A = new int[2];        A[0] = A[1] = -1;        for(int i = 0; i
     
       map = new HashMap
      
       ();    for (int i = 0; i < numbers.length; i++) {        if (map.containsKey(target - numbers[i])) {            result[1] = i ;            result[0] = map.get(target - numbers[i]);            return result;        }        map.put(numbers[i], i );    }    return result;}}